Portable data processing apparatus, such as workstations and other devices, may be connected to a local area network (LAN) via wireless links, which may use infrared or radio signals as a communications medium. The connection of the portable data processor to the LAN is through a base station. The base station is an intelligent device that may appear to the LAN as a bridge, a gateway or a concentrator. There may be multiple mobile units connected by wireless links to a single base station, the base station providing a protocol that enables the mobile units to communicate with the LAN. The mobile units may appear to the LAN to be a subnet or to be part of the same logical LAN. Whatever protocol is employed, a mapping exists for ensuring that network data messages, or datagrams, that are addressed to a mobile unit are routed to the appropriate base station for transmission to the addressed mobile unit. The mapping also ensures that datagrams originating with a particular one of the mobile units are correctly routed into the LAN.
If multiple infrared base stations are employed to provide coverage of a large space, such as a corridor, an auditorium, or an open-plan office, overlapping coverage is desirable to avoid `dead spots` where no communication is possible. However, it then becomes likely that there will exist overlapping regions wherein a given mobile unit is within range of two or more base stations. In radio cellular telephone systems this problem is resolved by coordinated frequency re-use. However, for an infrared transmission medium the use of alternate frequencies or wavelengths is severely restricted by the unavailability of low cost, tunable infrared sources.
In U.S. Pat. No. 4,456,793, issued June 26, 1984, W. E. Baker et al. describe a cordless telephone system having infrared wireless links between handsets and transponders. The transponders are wired to subsystem controllers which are in turn wired to a system controller. The central controller polls the cordless stations every 100 milliseconds to detect cordless station locations and to identify "missing" cordless stations.
In U.S. Pat. No. 4,807,222, issued Feb. 21, 1989 N. Amitay describes a LAN wherein users communicate with RF or IR signals with an assigned Regional Bus Interface Unit (RBIU). Protocols such as CSMA/CD and slotted ALOHA are employed in communicating with the RBIUs.
In commonly assigned U.S. Pat. No. 4,809,257, issued Feb. 28, 1989, D. Gantenbein, F. Gfeller and E. Mumprecht describe an infrared communication system are assigned to one of a plurality of hierarchical categories.
In commonly assigned U.S. Pat. No. 4,402,090, issued Aug. 30, 1983, F. Gfeller et al. describe an infrared communication system that operates between a plurality of satellite stations and a plurality of terminal stations. A host computer communicates with the terminal stations via a cluster controller and the satellite stations, which may be ceiling mounted. Communication with the terminal stations is not interrupted even during movement of the terminal stations.
In IBM Technical Disclosure Bulletin, Vol. 20, No. 7, December 1977, F. Closs et al. describe the use of both line-of-sight and diffuse transmission of infrared signals for wireless communications between a ceiling-based controller and a plurality of terminals.
In IBM Technical Disclosure Bulletin, Vol. 24, No. 8, page 4043, January 1982, F. Gfeller describes general control principles of an infrared wireless network incorporating multiple ceiling mounted transponders that couple a host/controller to multiple terminal stations. Access to the uplink channel is controlled by a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method.
It is thus an object of the invention to provide method and apparatus to coordinate the usage of a wireless transmission medium amongst a plurality of base stations having overlapping coverage areas.
It is a further object of the invention to provide method and apparatus for synchronizing the base stations to a slotted transmission protocol.